Solution for etching copper surfaces and method of depositing metal on copper surfaces

ABSTRACT

A solution for etching copper or a copper alloy for producing a copper surface having the brightest possible finish for a metallization that is to follow is described. The solution has a pH on the order of 4 or less and is free of sulfate ions. It comprises: a) at least one oxidizing agent selected from the group comprising hydrogen peroxide and peracids, b) at least one substance selected from the group comprising aromatic sulfonic acids and salts of the aromatic sulfonic acids and optionally c) at least one N-heterocyclic compound. Further a method for depositing metal onto the surface of copper or a copper alloy is described. Said method comprises the following method steps: a) contacting the surface with the solution in accordance with the invention and b) coating the surface with at least one metal. The solution and the method are especially suited in the production of electric circuit carriers, more specifically for semiconductor manufacturing.

SPECIFICATION

The invention relates to a solution for etching copper or of a copperalloy and to a method of depositing metal onto the surface of copper ora copper alloy, said surface having been previously etched using saidsolution. Both the solution and the method preferably serve to producecircuit carriers, more specifically printed circuit boards, or aresuitable for the semiconductor technique, as well as to produceleadframes and contacts such as e.g., multipoint connectors and contactsin switches, plug and socket connectors, sockets and plugs.

During the manufacturing of circuit carriers, copper surfaces aretreated, inter alia, with an etch solution to remove contaminants on thesurface or to prepare them for the treatment that is to follow. Onethereby aims at removing from 1-2 μm copper. In the followingtreatments, organic or metal layers are then applied. These layers mayperform different functions. Metal layers for example can formsolderable or bondable regions or serve as an etch resist. Depending onthe purpose they serve, these layers may remain either permanently ortemporarily on the copper surface.

In any event, in order to prevent parts of the surface layer fromstripping or chipping off, it is necessary that a strong adherent bondforms between the treated copper surface and the layer to be applied.

In the manufacturing of semiconductors, the phenomenon of epoxy bleedout could be observed at the surface of leadframes, said phenomenonoccurring for example after the chip has been resin coated, when theexcess epoxy resin leaks (“bleeds out”) onto the external contacts ofthe leadframe. These sites are then lost to electroplating.

In practice, known etch solutions for slightly etching copper surfacesin view of a metallization to follow often consist of aqueous solutionsof alkali peroxodisulfate or of hydrogen peroxide, each combined withsulfuric acid.

These etchants permit to achieve etch textures with residual roughnessof 1-2 μm which results in a size increase of the surface of up to 50%as compared to the geometrical surface. This surface increase is due tothe coarse copper texture obtained.

Further known copper etchants are iron(III) salts, hypochlorite andcopper(II) salts in an aqueous acidic or alkaline solution; however,these are hardly considered for the application described herein abovebecause their etch effect is too strong, which makes them more suitedfor use for differential etching or for completely removing copper onbase material. In these applications, the undercuts (steepness of theslope of the conductor lines for example) must be considered, whereas inthese cases the etch textures are not important. The etch resist must beprevented from being attacked and it must be possible to regenerate theetch solution (copper recovery) without any problem.

The composition of the etch solution depends, inter alia, on the type oflayer to be applied thereon. Metallic and organic layers for examplepose different demands on the structure and quality of the coppersurface.

Metallic bright copper surfaces are not considered to be suited forbeing subsequently coated with organic coatings such as photoresists orsolder resist masks nor are they suited for press-laminatingmultilayers. For this reason, modified etchants are used to roughen thecopper and to simultaneously form a brown-black copper(I)/copper(II)oxide film. The adhering layers of the photoresists and the solderresist masks or of epoxy resin can become anchored in said oxide filmduring press-laminating (U.S. Pat. No. 6,036,758; EP 0 442 197 A2; EP 0926 265 A1). Said etchants contain mineral acids, alkane sulfonic acids,mixtures of the acids mentioned, inhibitors and known oxidizing agentsand are intended to oxidize and roughen smooth copper areas. They arenot suited for subsequent metallization though.

The increasing miniaturization of circuit carriers, and morespecifically miniaturization in semiconductor processing, calls for newsolutions. Wet processes, more specifically electroplating processes,such as copper plating and the manufacturing of copper conductors onmicrochips are finding increasing acceptance in this technical field. Inthis field, the surfaces are required to be polished and bright, anessential prerequisite for obtaining fine circuit structures. Thesepolished surfaces can be produced using what are termed etch (polishing)solutions. Such type solutions are known from EP 1 167 585 A2, U.S.Patent Application No. 2002/0022370 and U.S. Pat. No. RE 37,786 E andresult in high polish copper surfaces.

Apart from the oxidizing agent, hydrogen peroxide or alkaliperoxodisulfate, the composition and effect of these etch (polishing)solutions considerably differ from the etch solutions mentioned hereinabove. In addition to considerable amounts of abrasive substances(silicon dioxide—aluminum dioxide—ceroxide—zirconium dioxide) includedfor increasing the removal rates, the solutions are disclosed to containviscosity increasing agents (polyalcohols, polyethylene glycols, etc.),organic acids (amino acetic acid, amido sulfuric acid, oxalic acid,citric acid, gluconic acid), inorganic acids and inhibitors (N-methylformamide, benzotriazole, imidazole, phenacetine, thiourea,mercaptobenzothiazole).

Moreover, the pH of the solutions varies from slightly acid to stronglyalkaline (9-14, 3-10, 5-8) thus clearly exceeding that of currentetchants. The solutions mentioned also serve to etch, slightly etch orlevel copper surfaces, with only low removal rates of from 0.03 to 0.1μm being expected for polishing in accordance with the intended purpose.Higher removal rates may for example be achieved by increasing thetemperatures and by adding abrasive substances. In this case however,the amount of added complexing agents has to be increased in order tokeep the oxidized copper in solution as oxidized copper, because of thehigh pH of the solution, can only be kept in solution using complexingagents such as EDTA or NTA for example. If the etch solution containstoo much copper, it cannot be used any longer.

DE-OS 21 49 196 discloses an aqueous solution for etching copper or acopper alloy, the solution comprising peroxodisulfate and at least oneheterocyclic azol compound. Further this solution may also contain oneor more halides and sulfuric acid or phosphoric acid. This solution isintended to be used for etching copper on printed circuit boardmaterial.

DE 100 34 022 A1 discloses an acid treatment solution for coppersurfaces, the solution comprising hydrogen peroxide, at least onefive-membered heterocyclic compound, such as a tetrazol, and at leastone microstructuring agent, this agent being selected from the groupcomprising organic thioles, sulfides, disulfides and thioureas. Thesolution may further contain an acid, sulfuric acid for example. Thecopper surfaces produced by a treatment with this solution have amicrostructure having two roughness values, the first one being in therange of from 1 to 10 μm and the second one being in the range of from50 to 500 nm. The solution is intended to be used for the manufacture ofprinted circuit boards.

U.S. Pat. No. 6,036,758 describes an etch solution for the surfacetreatment of copper that comprises hydrogen peroxide and an aromaticsulfonic acid or a salt thereof. In addition, this etch solutionincludes, inter alia, an inorganic acid, with sulfuric acid comprised ina range between 2 and 20% (w/v) being preferred, a concentration between5 and 10% (w/v) being particularly preferred.

EP 1 167 482 A2 discloses an etch (polishing) solution comprising anN-heterocyclic compound, hydrogen peroxide and a salt of the dodecylbenzene sulfonic acid. The solution is suited for manufacturingintegrated circuits, the excess wiring material being intended to beetched away after the metal plating step. In addition to theconstituents mentioned, abrasive substances and additives such assulfuric acid for example, may be added to the etch (polishing)solution. The removal rate is a function of the pH, the temperature andthe kind and strength of the acids used. The solution operates at a pHof between 5 and 12.

In the printed circuit board technique, miniaturization poses newdemands on the surfaces, more specifically on the copper surfaces, whichmay for example form the basis for end layers such as electrolessnickel-gold, chemical tin, silver, palladium and combinations of themetals mentioned. For forming fine circuit structures, the surfacesproduced must provide good bonding for subsequent metallization.

Bonding strength is determined, inter alia, by the surface structure ofthe etched copper, whereby it is assumed that the achieved bondingstrength will be the greater the coarser the surface structure of thecopper surface is. As a result, it is expected that bright coppersurfaces will have a reduced bonding strength.

It partially appeared that the use of the etchants mentioned hereinabove tends to cause problems during metal deposition, for exampleduring electroless deposition of copper, tin, silver, nickel, gold,palladium and bismuth. These problems are that in some cases the metalsmentioned could not deposit at all and that in some other cases but avery non-uniform metallization could be obtained.

However, defect bondings of the metal layers to be applied do not allowto dispense with slightly etching the copper.

In the course of the tests performed with etchants that produce brightcopper surfaces, it has been found out that these suffer fromconsiderable shortcomings. After etching, dark brown stains form on thesubstrate being taken to the rinse tank, said stains being dull oncethey have been dipped in sulfuric acid. The operating range of theetchants is very narrow, adding tremendous expense to the analysis ofthe bath to be operated.

Using the known methods and processing solutions, it is therefore notpossible to concurrently meet the following requirements:

-   -   i) Providing metal layers with sufficient bonding strength, even        on very narrow copper conductors that are applied to substrates,        such as printed circuit board material, leadframes or various        contacts. This requirement is intended to prevent the metal        deposits from chipping off or from being otherwise stripped off.    -   ii) Providing a copper surface as bright as possible for the        metallization to follow, in particular in order to permit        formation of very fine circuit structures.    -   iii) Ensuring sufficient process safety as the substrate is        being taken to the rinse tank so that dark brown stains are        prevented from forming on the copper surface.

Besides these requirements, the etched surfaces on the leadframes areintended to additionally minimize the formation of resin smear as aresult of the observed epoxy bleed out or to largely prevent said resinsmear from contaminating the external contacts. Furthermore, theetchants are intended to be affordable and easy to handle.

Therefore the basic object of the present invention is to meet therequirements mentioned in order to overcome the drawbacks of the knownsolutions and methods. More specifically, it is intended to produce thebrightest possible copper surface.

The solution to these problems is achieved by the solution for etchingcopper or a copper alloy in accordance with claim 1, the use of thesolution in accordance with claim 12 and the method for depositing metalin accordance with claim 13. Preferred embodiments of the invention arerecited in the subordinate claims.

The solution in accordance with the invention serves to etch copper or acopper alloy on substrates, preferably on electric circuit carriers,more specifically on printed circuit boards or in the semiconductortechnique, as well as to produce leadframes and contacts such as e.g.,multipoint connectors and contacts in switches, plug and socketconnectors, sockets and plugs. The etch solution preferably serves toproduce a copper surface that is suited for being subsequently coatedwith metals. The solution in accordance with the invention is a solutionhaving a pH of about 4 and lower. It does not contain sulfate ions. Itcomprises:

-   -   a) at least one oxidizing agent, selected from the group        comprising hydrogen peroxide and peracids, and    -   b) at least one substance, selected from the group comprising        aromatic sulfonic acids and salts of the aromatic sulfonic        acids.

Due to their manufacturing method, organic and especially aromaticsulfonic acids have residual concentrations in sulfate ions. This is dueto the fact that the sulfonic acids are subject to hydrolysis thusforming the respective non-sulfonated aromatic compounds and sulfateion. This reaction is the reversal of the sulfonation reaction.Therefore aromatic sulfonic acids are always accompanied by sulfate ions(Ullmanns Encyclopädie der technischen Chemie [Ullmann's encyclopedia oftechnical chemistry], 4^(th) edition, volume 8, pages 412-416).Especially aromatic sulfonic acids that contain a group, such as a nitroradical, in a meta-position at the aromatic ring are preferably subjectto such hydrolyzation. In particular this applies tom-nitrobenzenesulfonate which is expressis verbis mentioned in U.S. Pat.No. 6,036,758 for its use as an additive to a composition useful for thesurface treatment of copper. Therefore it is considered that thesolutions disclosed in this document are not free of sulfate as requiredaccording to the present invention.

Hereto before, the circumstance that aromatic sulfonic acids containsulfate ions has generally been disregarded because, for etching,sulfonic acids are often used precisely together with sulfuric acid. Ithas however been found out that bright copper layers can only beobtained with sulfate free sulfonic acid in combination with the otherconstituents of the solution in accordance with the invention.

By the sulfate ion free solution in accordance with the invention asolution is meant that has a sulfate ion concentration of less than 0.2%(w/v). It is more preferred to have an even lower concentration ofsulfate ion in the solution, which concentration corresponds to thatconcentration of sulfate ion in the solution established if sulfate ionis brought into the solution by adding sulfate containing sulfonic acidor the salt thereof to be used in accordance with this invention to thesolution wherein the sulfate ion concentration in the sulfonic acid orthe salt thereof is less than 0.2% (w/v). In both definitions of sulfatefree solution sulfate ion may originate from sulfonic acid, the saltthereof and/or from any other source.

For quantification of the concentration of sulfate ion in a sulfonicacid or in the salt thereof or in the solution according to the presentinvention, known methods may be utilized such as ion chromatography orthe method for gravimetric determination of sulfate ion as the bariumsulfate. If the concentration of sulfate ion in the solution is higherthan the value given above, its concentration must be reduced to lessthan this value, for example by precipitating sulfate ion as bariumsulfate by using barium chloride. The aromatic sulfonic acids themselvesdo not form hardly soluble barium salts. Therefore the aromatic sulfonicacids do not precipitate and sulfate can be easily separated from thesolution. During the use of such solution, for maintaining the solutionfree of sulfate, the solution may be regenerated with a barium saltsolution, e.g. a solution of barium carbonate. Such regeneration may beperformed by subjecting the solution to a barium salt solution in abatch operation. Alternatively regeneration can also be performedcontinuously by adding the barium salt in a small excess to the solutionaccording to the invention in order to continuously remove sulfatethereof by precipitation.

The method in accordance with the invention is simple, easy to performand cheap. It serves to prepare the copper or copper alloy surface tothereafter deposit metal to this surface, especially of a copper orcopper alloy layer applied to a substrate. It involves the followingmethod steps:

-   -   a) contacting the surface with the solution in accordance with        the invention and thereafter    -   b) coating the surface with at least one metal.

The substrates comprising copper surfaces more specifically includeelectric circuit carriers, leadframes and contacts such as e.g.,multipoint connectors and contacts in switches, plug and socketconnectors, sockets and plugs.

The solution in accordance with the invention has a micro-etchingeffect. At 23° C., the etch rate is on the order of 1-2 μm per minute.The solution in accordance with the invention permits to obtain anexcellent quality, oxide free, smooth, salmon-pink copper surface with asatin to bright finish. This appearance is due to the obtained verysmall copper crystals that form a uniform microstructure on the surfaceof the metal, thus being responsible for the superior opticalappearance. The microstructure of metal surfaces may for example beanalyzed using Atomic Force Microscopy (AFM), which concurrently permitsto determine the residual roughness of the surface. AFM measurementsrely on measuring the strength and contributions of interparticularinteractions such as Van der Waals interactions or electrostaticinteractions and permit to image surface structures at the atomic level.They show that, after using the solution in accordance with theinvention, a residual roughness R_(max) of 26 nm was achieved. Ascompared to the geometrical area, the surface only increased by 3.6%(measured on 10 μm by 10 μm areas).

It has been found out that the values obtained for the residualroughness are well correlated with the optical appearance of the metalsurface. Table 1 opposes the values obtained for the residual roughnessand the optical appearance. TABLE 1 Residual roughness R_(max)[nm]Optical Appearance <40 bright finish 41-50 satin to bright finish 51-79satin finish >80 matte

Moreover, it has been found out that the residual roughness depends onthe acidic strength of the solution in accordance with the invention. Itcan be noted that higher acidic strength leads to greater residualroughness and, as a result thereof, to a greater increase of the surfaceand vice versa. The acidic strength may also influence the etch ratewhich in turn affects the residual roughness with higher etch ratesusually resulting in comparatively greater residual roughness. In viewthereof, it has also been found out that the etch rate is not the solefactor responsible for the formation of surfaces having the brightestpossible finish. It was clearly to be seen that the use of the morespecifically sulfate free solution in accordance with the inventionproduced, even at the same etch rate, a significantly enhancedappearance i.e., a surface with a brighter finish.

The solution and the method in accordance with the invention permit toeliminate the problems arising using the known means. The brightsurfaces required as a result of the ever-increasing miniaturization canbe produced with the solution in accordance with the invention withoutthe trade-off of a simultaneous reduction of bonding strength of themetallization to follow. There is no formation of brown stains on thebright surface while the substrate is being taken to the rinse tank,which additionally further improves the bonding strength.

Surprisingly, resin smear, which may form on the areas of contact ofleadframes as a result of “epoxy bleed out” as they are being cast inplastic, may be minimized using the solution in accordance with theinvention prior to electrolytically coating with solder metal.

Another advantage of a metallic bright surface showing no stains is thatone avoids the problems stemming from a matte and/or stained surfaceduring adjustment of a photomask on the conductive pattern. Adverseeffects during the following metal plating step, for example duringelectroless deposition of bismuth, copper, tin, silver, nickel, gold orpalladium are not observed.

The solution in accordance with the invention also permits to overcomethe problem arising using known solutions, the problem being that insome cases there is no metal deposit at all though the surface haspreviously been etched with the etch solution or that the depositobtained is non-uniform. These problems arising using the known etchsolutions are due to the fact that the copper surfaces produced have toocoarse a structure i.e., too high roughness depth values. If one assumesfor example that the copper surfaces have a roughness depth of 1-2 μm,the subsequent layers to be formed thereon, which usually have athickness of 0.2-5 μm, will adapt to the given texture. The epitaxialeffects resulting from the structure can be demonstrated. Theynegatively affect corrosion behavior of chemically deposited nickel forexample. For the reasons mentioned, surfaces that are etched in thismanner are not suited for producing fine circuit structures.

The resulting copper removal is low so that after etching the thicknessof the copper layer varies within but a narrow range. Another advantageis the low pH of the solution, which is 4 or less, as such a pH permits,in contrast to many prior art etch (polishing) solutions, to utilize thesolution while using alkaline soluble solder resists. The service lifei.e., the copper absorption of such an etchant without the trade-off ofa reduction of the brightness of the copper surfaces, is on the order of20 g/l and more, which is also due to the low pH without complexingagents having to be added to the solution. The operation of the bath,the analyses and the necessary replenishments are easy to perform.

The solution in accordance with the invention has a pH on the order of 4and less, preferably of 3 and less. A pH ranging from 2.2 to 1.8 isparticularly preferred.

In order to achieve the etching effect of the solution of the inventionas it has been described, the aromatic part of the aromatic sulfonicacids and of the salts of the aromatic sulfonic acids preferablycomprises at least one phenyl group, which may be substituted by one ormore radicals selected from the group comprising nitro, amino, hydroxy,halogen, C₁-C₅-alkyl radicals and C₁-C₅-alkoxy radicals. The alkyl andalkoxy radicals may also be substituted, preferably by amino, hydroxyand/or halogen. If the phenyl group is substituted by a plurality ofradicals, these may be selected independently from one another.Compounds which are selected from the group comprising benzene sulfonicacid, phenol sulfonic acid, toluene sulfonic acid and amino benzenesulfonic acid are particularly preferred. Naphthalene sulfonic acid isanother preferred substance. The aromatic sulfonic acids of particularpreference are such having a comparatively low acidic strength.

The solution may further preferably contain at least one N-heterocycliccompound. The N-heterocyclic compounds are preferably selected from thegroup comprising mono-N, di-N, tri-N and tetra-N heterocyclic compounds.More specifically, the compounds may thereby have 5 or 6 members.Compounds of the group comprising pyridine, N-methylpyrrolidone,adenine, guanine, uric acid, imidazole, pyrazole, piperazine,pyrrolidone, pyrroline, triazole, tetrazole and the derivatives thereofare preferably suited.

The N-heterocyclic compounds contained in the solution in accordancewith the invention do not substantially influence the etch rate of thesolution.

The preferred concentration ranges of the solution constituents are asfollows:

aromatic sulfonic acids and the salts of the aromatic sulfonic acids:

-   -   preferably from 2 to 250 g/l,    -   more preferably from about 20 to about 60 g/l,        N-heterocyclic compounds:    -   preferably from 0.1 to 300 g/l,    -   more preferably from about 10 to about 80 g/l,        hydrogen peroxide (35% (w/v)):    -   preferably from about 60 to about 110 g/l,    -   more preferably from about 80 to about 100 g/l, most preferably        about 100 g/l.

It is to be understood that the etch solution also operates outside ofthe concentration ranges indicated. Accordingly, the concentrationranges indicated are mere standard values.

The peracids used are selected from the group comprising organic andinorganic peracids, said peracids being preferably selected from thegroup comprising perboric acid and perbenzoic acid.

In addition to the constituents mentioned, the solution in accordancewith the invention may comprise at least one adjuvant selected from thegroup comprising polyethylene glycol, polypropylene glycol and thederivatives thereof. It could be observed that, by adding this adjuvant,the copper crystals became even smaller, which additionally minimizesthe size increase of the surface, thus further reinforcing the brightfinish of the surface treated. Said adjuvants preferably have a degreeof polymerization in the range of from about 100 to about 1000.

According to the method of the invention, the copper surfaces arepreferably cleaned prior to being treated with the solution inaccordance with the invention in order to remove from the copper surfacecontaminations that would interfere with the treatment. Conventionalacidic cleaning fluids may be utilized. Usually, surfactants and, ifneed be, complexing agents such as triethanolamine are added to theaqueous cleaning fluids in order to improve the cleaning effect.

Preferably, a rinse step with deionized water for example may beprovided for after cleaning.

Then, the copper surfaces are treated with the solution in accordancewith the invention, said solution being preferably operated in themethod at a temperature of from about 20° C. to about 60° C. Theprocessing time preferably amounts to from about 10 sec to about 400sec. The higher the temperature of the solution during etching, thefaster the etch reaction will proceed. Accordingly, a shorter processingtime is needed in this case to obtain a certain etch result. Fortechnical reasons, an etch temperature ranging from about 20 to about25° C. is preferred to keep easy control of the method, with thepreferred processing time being about 120 sec.

Next, the copper surfaces can be contacted with sulfuric acid, morepreferably with 1% (w/v) sulfuric acid. Prior to metal plating, thesurfaces can preferably be rinsed, more specifically using deionizedwater. The metal deposited is preferably selected from the groupcomprising bismuth, copper, tin, gold, silver, palladium and nickel,with the metal being more preferably formed as electroless nickel-goldor as chemical tin.

The metal layers applied may for example serve as bondable andsolderable contact areas or as electrical contact layers for pushbuttons or plug contacts. The metal layers may for example be depositedby electrochemical, electroless or chemical plating. Chemical depositionby metal to metal charge transfer is preferred, one metal (here copperor a copper alloy) dissolving partially while the dissolved metal,chemical tin for example, is deposited. Electroless plating e.g.,electroless nickel-gold is also preferred.

For forming an electroless nickel-gold layer, the copper surface isfirst treated with a bath by means of which palladium nuclei aredeposited onto the surface. Next, metal plating may be performed inanother bath comprising nickel ions, for example in the form of asulfate salt, and a reducing agent. Usually, the reducing agent utilizedis a hypophosphite salt, for example the sodium salt thereof, or thecorresponding acid thereof. In this case, a nickel-phosphorus layerforms. If a nickel-boron coating is to be generated, the reducing agentutilized will be a borane, for example dimethylamine borane or aboranate such as sodium boron hydride. If pure nickel layers are to bedeposited, the reducing agent used will preferably be hydrazine or aderivative thereof. These baths additionally comprise complexing agents,more specifically organic carboxylic acids, pH adjusting agents such asammonium hydroxide or acetate, as well as stabilizers such as sulfurcompounds or lead salts. The gold layer is applied to the electrolessplated nickel layer, for example by a charge transfer method or byelectroless plating, i.e., using a reducing agent.

For forming a chemical tin layer, the copper surface is contacted with asolution comprising tin(II) ions, for example tin(II) sulfate, an acidsuch as sulfuric acid, and a thiourea derivative. The tin layer isformed on the copper surface-via a charge transfer reaction, with copperdissolving to the benefit of tin.

The substrates comprising the copper surfaces can be processed incurrent dip plants. For the processing of printed circuit boards, it hasbeen found out that the utilization of what are termed conveyorizedplating lines in which the printed circuit boards are conveyed throughthe plant on a horizontal conveying direction (path) while beingcontacted with the processing fluids via suited nozzles such as spray orflow nozzles is particularly advantageous. For this purpose, the printedcircuit boards can be held horizontally or vertically or in any otherorientation.

In a similar fashion, substrates comprising copper surfaces such asleadframes can be processed in reel-to-reel (RTR) systems.

The following examples serve to further explain the invention:

Except for Comparative Example 4a, the concentration of the sulfate ionsin all of the etch solutions of the examples described was reduced to avalue that corresponds to a concentration of sulfate ion in the sulfonicacid of less than 0.2% (w/v) which is added to the solution. This wasachieved by means of barium sulfate precipitation. The barium sulfatethus obtained was filtered away.

The pH in the solutions according to the present invention was alwaysabout 4 or less.

In the examples, printed circuit boards that were electrolyticallyreinforced with copper cladding were processed in the following manner:

-   -   1. Cleaning in an acidic, conventional cleaning fluid;    -   2. Rinsing in water;    -   3. Processing the printed circuit boards for 2 minutes each (see        examples);    -   4. Dipping into a 1% (w/v) sulfuric acid;    -   5. Rinsing in water;    -   6. Metal plating with electroless Ni/P and then with chemical        gold using current metal plating solutions.

EXAMPLE 1a Example in Accordance with the Invention

An aqueous solution was prepared by mixing the following constituents:phenol-4-sulfonic acid (sulfate free)  50 g hydrogen peroxide 35% (w/v)100 g

Deionized water was added to bring the volume to 1 L.

The solution was heated to 23° C. Then, two copper foils (printedcircuit board quality) were each treated according to the aforementionedmethod scheme by dipping them for 120 sec into the solution. Aftertreatment with warm deionized water, they were dried. One foil was usedto determine the residual roughness while the other foil was metalplated in compliance with the method scheme.

The copper layer of the analyzed foil had a satin to bright finish andhad a residual roughness of 44 nm. No stains formed while it was takento the rinse tank. The etch rate was 0.66 μm/min. After tin plating, anadherent bond between the copper surface and the applied tin layer couldbe observed.

EXAMPLE 1b Example in Accordance with the Invention

Example 1a was repeated using a solution with the following composition:toluene-4-sulfonic acid (sulfate free)  25 g hydrogen peroxide 35% (w/v)100 g

Deionized water was added to bring the volume to 1 L.

The copper layer had a satin finish and a residual roughness of 63 nm.No stains formed while it was taken to the rinse tank. The etch rate was2.0 μm/min. After silver plating, a sufficiently adherent bond betweenthe copper surface and the applied silver layer could be observed.

As mentioned herein above, it appeared that the etch rate is a functionof the acidity of the acid used as can be seen from the Examples 1a and1b. Phenol-4-sulfonic acid has a lower acidity and accordingly had amuch lower etch rate than toluene-4-sulfonic acid. This, together withthe formation of smaller copper crystals, resulted in a reduced residualroughness in Example 1a.

EXAMPLE 2 Example in Accordance with the Invention

Example 1b was repeated using a solution with the following composition:toluene-4-sulfonic acid (sulfate free) 25 g hydrogen peroxide 35% (w/v)100 g  polyethylene glycol  25 ml (degree of polymerization: 100-400)

Deionized water was added to bring the volume to 1 L.

The copper layer had a satin to bright finish and had a residualroughness of 43 nm. No stains formed while it was taken to the rinsetank. The etch rate was 1.8 μm/min. After nickel plating, an adherentbond between the copper surface and the applied nickel layer could beobserved.

By adding polyethylene glycol in Example 2, one obtained a reduced etchrate and, as a result thereof, a reduced residual roughness whendirectly compared with Example 1b.

EXAMPLE 3 Example in Accordance with the Invention

Example 1b was repeated using a solution with the following composition:toluene-4-sulfonic acid (sulfate free) 25 g hydrogen peroxide 35% (w/v)100 g  pyridine  30 ml

Deionized water was added to bring the volume to 1 L.

The copper layer was bright and had a residual roughness of 31 nm. Nostains formed while it was taken to the rinse tank. The etch rate was1.5 μm/min. After electroless palladium plating, an adherent bondbetween the copper surface and the applied palladium layer could beobserved.

The solution in accordance with the invention was prepared, using thesame sulfonic acid as in Example 1b and adding an N-heterocycliccompound. As compared to Example 1b, the etch rate diminished and thesize of the copper crystals was strongly reduced. As a result, theresidual roughness dropped significantly and the surface accordingly hada uniform bright appearance.

COMPARATIVE EXAMPLE 4a

Example 1a was repeated using a solution with the following composition:benzene sulfonic acid (containing about 2% (w/v) H₂SO₄) 25 g hydrogenperoxide 35% (w/v) 100 g  pyridine  30 ml

Deionized water was added to bring the volume to 1 L.

The copper layer was matte and had a residual roughness of 88 nm. Nostains formed while it was taken to the rinse tank. The etch rate was1.2 μm/min at a temperature of 25° C. After bismuth plating, an adherentbond between the copper surface and the applied bismuth layer could beobserved.

Here it appeared that, although the etch rate was low, the surface thatformed was not bright. The etch texture could not be improved by addingan N-heterocyclic compound. This example permitted to demonstrate theinfluence of a sulfonic acid comprising sulfate ions that was not inaccordance with the present invention. The solution behaved like asulfuric acid etch solution.

EXAMPLE 4b Example in Accordance with the Invention

Comparative Example 4a was repeated using a solution with the followingcomposition: benzene sulfonic acid (sulfate free) 25 g hydrogen peroxide35% (w/v) 100 g  pyridine  30 ml

Deionized water was added to bring the volume to 1 L.

The copper layer had a satin to bright finish and had a residualroughness of 45 nm. No stains formed while it was taken to the rinsetank. The etch rate was 1.3 μm/min. After chemical gold plating, anadherent bond between the copper surface and the applied gold layercould be observed.

This example clearly shows the positive effect the solution inaccordance with the invention comprising a sulfate free sulfonic acidhas on the etch texture when directly compared with Comparative Example4a. The etch rate was similar, but the residual roughness could bereduced by almost half which resulted in an optically significantlyenhanced surface.

It is to be understood that various modifications and substitutions bytechnical means may be applied to what has been described by way of theexamples hereinabove without departing from the scope of the inventionas defined by the appended claims. It is further understood thatcombinations of features described in this application will be suggestedto persons skilled in the art and are to be included within the purviewof the described invention and within the scope of the appended claims.All publications, patents and patent applications cited herein arehereby incorporated by reference.

1. A solution for etching copper or a copper alloy, said solution havinga pH on the order of 4 or less, comprising: a) at least one oxidizingagent selected from the group comprising hydrogen peroxide and peracidsand b) at least one substance selected from the group comprisingaromatic sulfonic acids and salts of the aromatic sulfonic acidscharacterized in that the solution is free of sulfate ions.
 2. Thesolution according to claim 1, characterized in that it furthercomprises at least one N-heterocyclic compound.
 3. The solutionaccording to claim 2, characterized in that the concentration of theN-heterocyclic compounds is in the range of from about 0.1 to about 300g/l.
 4. The solution according to any one of claims 2 and 3,characterized in that at least one N-heterocyclic compound is selectedfrom the group comprising mono-N, di-N, tn-N and tetra-N heterocycliccompounds.
 5. The solution according to any one of claims 2,characterized in that at least one N-heterocyclic compound is selectedfrom the group comprising pyridine, N-methylpyrrolidone, adenine,guanine, uric acid, imidazole, pyrazole, piperazine, pyrrolidone,pyrroline, triazole, tetrazole and the derivatives thereof.
 6. Thesolution according to any one of the preceding claims 1-3, characterizedin that the concentration of the substances is in the range of fromabout 5 to about 250 g/l.
 7. The solution according to any one of thepreceding claims 1-3, characterized in that at least one salt of thearomatic sulfonic acids is selected from the group comprising sodium andpotassium salts.
 8. The solution according to any one of the precedingclaims 1-3, characterized in that the aromatic part of at least onearomatic sulfonic acid or of at least one salt of the aromatic sulfonicacids comprises at least one phenyl group.
 9. The solution according toclaim 8, characterized in that at least one phenyl group is substitutedby one or more radicals selected from the group comprising nitro, amino,hydroxy, halogen, C₁-C₅-alkyl radicals and C₁-C₅ alkoxy radicals. 10.The solution according to any one of the preceding claims 1-3,characterized in that at least one aromatic sulfonic acid is selectedfrom the group comprising benzene sulfonic acid, phenol sulfonic acid,toluene sulfonic acid, amino benzene sulfonic acid and naphthalenesulfonic acid.
 11. The solution according to any one of the precedingclaims 1-3, characterized in that the solution comprises at least oneadjuvant selected from the group comprising polyethylene glycol,polypropylene glycol and the derivatives thereof.
 12. Use of thesolution according to any one of claims 1-3 for producing electricalcircuit carriers or for the semiconductor technique in vertical and/orhorizontal lines or for producing leadframes in RTR systems or forproducing multipoint connectors and contacts in switches, plug andsocket connectors, sockets and plugs.
 13. A method for depositing metalto the surface of copper or a copper alloy, said method comprising thefollowing method steps: a) contacting the surface with the solution inaccordance with any one of claims 1-3 and b) coating the surface with atleast one metal.
 14. The method according to claim 13, characterized inthat the substrate is selected from the group comprising electricalcircuit carriers, leadframes, multipoint connectors and contacts inswitches, plug and socket connectors, sockets and plugs.
 15. The methodaccording to claim 13, characterized in that the substrate is contactedwith an acidic cleaning fluid prior to method step a).
 16. The methodaccording to claim 13, characterized in that the substrate is contactedwith sulfuric acid prior to method step b).
 17. The method according toclaim 13, characterized in that the metal is selected from the groupcomprising copper, tin, gold, silver, palladium, bismuth and nickel. 18.The method according to claim 17, characterized in that the metal iselectroless nickel-gold or chemical tin.
 19. The method according toclaim 13 for producing electrical circuit carriers or for thesemiconductor technique in vertical and/or horizontal lines or forproducing leadframes in RTR-systems.